1. Field of the Invention
The invention relates to a resistor for dividing an applied voltage into an intermediate voltage being below the applied voltage. The invention further relates to a cathode ray tube and an electrode gun.
2. Description of the Related Art
Such a resistor assembly is known from European patent application EP-A-36901. The resistor assembly described in this document is used in electron guns for cathode ray tubes (CRT). The known resistor assembly is mounted on the electron gun in the neck of the CRT. A first terminal of the resistor assembly is connected to a stem pin of the electron gun of the cathode ray tube, a second terminal of the resistor assembly is coupled to the anode of the cathode ray tube and a third terminal of the resistor assembly is connected to an intermediate grid of the electron gun of the cathode ray tube. The resistor assembly is used for supplying the intermediate voltage to the intermediate grid. This intermediate voltage is divided by the resistive voltage divider from the difference between the anode voltage and a ground or zero voltage. Normally, the voltage difference between the anode and the cathode is approximately 30 kV and the voltage difference between the potential of the intermediate grid and the potential of the cathode is approximately 15 kV. The intermediate voltage is defined by the ratio of the resistance of the first and second resistive layers. In order to obtain the intermediate voltage as a predetermined ratio of the anode voltage, the ratio of the resistance between the first and third terminal and the second and third terminal, respectively, is adjusted in a calibration step of the manufacturing process, for example, by selectively releasing one or more bridge connections in the additional resistive network.
Normally, the resistive layers are meandered or have a zig-zag shape. Design rules of the manufacturing process of the resistor assembly stipulate a minimum distance between adjacent branches of the resistive layers and also a maximum electric field strength per unit length of resistive layer. Furthermore, the resistor assembly has to fit in the neck of the CRT and connections have to be made between the third terminal and the intermediate grid of the CRT and between the second terminal and the anode of the CRT. Therefore, the resistor assembly normally has an elongated shape and its length is one of the factors that determine the length of the electron gun.
Furthermore, in the known resistor assembly, the resistive portions of the additional resistive network have approximately an identical resistance and, together with the bridge connections, then occupy a relatively large area of the resistor assembly in order to provide the assembly with a predetermined resistance value by selecting one or more resistive portions of the additional network. Manufacturing tolerances induce a deviation of the predetermined ratio of the first and second resistive layers. In order to obtain a predetermined ratio of the resistive voltage divider in a calibrating step, the actual ratio of the first and second resistive layers is measured and a predetermined resistance of the additional resistive network is selected by releasing one or more bridge connections to match the ratio of the series circuit of the first resistor together with the additional resistive network and the second resistor to the predetermined ratio of the resistive voltage divider. This additional resistive network occupies a relatively large area of the resistor assembly and determines, amongst other factors, the length of the resistor assembly.
It is an object of the invention to reduce the length of the resistor assembly without altering the current design rules of the manufacturing process. This object is achieved using first and second resistive portions having different resistance values, an optimal choice of the resistance values of the respective first and second portions can be made. As a result, a maximum range of predetermined values of the resistance of the additional resistive network can be obtained with a minimum number of resistive portions and bridge connections. By suitable choice of the resistance value of the first and second resistive portions, the area occupied by the additional resistive network on the substrate can be substantially reduced as compared to the area occupied by the resistive portions of the known resistor assembly, which portions have an equal size together with the bridge connections and define the same range of resistance values when the same design rules of the manufacturing process are applied. The length of the resistor assembly can thus be reduced. As a result, also the length of the electron gun and the complete cathode ray tube can be further reduced. This is an important advantage because the market demands shorter CRTs for use in televisions and computer monitors.
Furthermore, the predetermined value of the additional resistive network can be obtained by fewer releasing steps of the bridge connections, which saves time in the manufacturing process.
A particular embodiment of the resistor assembly according to the invention is characterized in that the resistance of the first resistive portion is twice that of the second resistive portion. By using a 1:2 ratio between the first and second resistive portions, a range of values 0, 1R, 2R or 3R can be obtained for a series circuit of the resistive portions, or a range of values 0, 2/3 R, R or 2 R can be obtained for a parallel circuit of the resistive portions.
A further embodiment of the resistor assembly according to the invention is characterized in that the additional resistive network comprises a third resistive portion which is releasably connected to the network terminals via a further bridge connection, and the ratios of the resistance of the first, second and third resistive portions are equal to 1:2:4. In this embodiment, a range of 7 values can be obtained in the range from 0,1,2,3 . . . 7 R for a series circuit of the resistive portions and a range of 0, 2/3R, R, 5/4 R, 4/3 R, 2R, 4R can be obtained for a parallel circuit of the resistive portions.
A further embodiment of the resistor assembly according to the invention is characterized in that the resistive layers, the resistive areas and the bridge connections comprise respective ruthenate lead systems of different ratios of lead and ruthenate, respectively.
A further embodiment of the resistor assembly according to the invention is characterized in that the resistor assembly comprises an insulating layer covering the first and the second resistor and the additional resistive network. For example, a high voltage glass having a relatively low melting point of, for example, 600xc2x0 C. can be applied across the resistive layers and the resistive areas.
It is a further object of the invention to provide an electron gun with a reduced length.
It is a further object of the invention to provide a cathode ray tube with a reduced length of the neck.